It is desirable in many polymerization processes, particularly a slurry phase or gas phase process, to use a supported catalyst system. Generally these catalyst systems include a metallocene and alumoxane supported on a carrier, such as silica. For example, U.S. Pat. No. 4,937,217 generally describes a mixture of trimethylaluminum and triethylaluminum added to an undehydrated silica to which a metallocene catalyst component is then added. EP-308177-B1 generally describes adding a wet monomer to a reactor containing a metallocene, trialkylaluminum and undehydrated silica. U.S. Pat. Nos. 4,912,075, 4,935,397 and 4,937,301 generally relate to adding trimethylaluminum to an undehydrated silica and then adding a metallocene to form a dry supported catalyst system. U.S. Pat. No. 4,914,253 describes adding trimethylaluminum to undehydrated silica, adding a metallocene and then drying the resulting supported catalyst system with an amount of hydrogen to produce a polyethylene wax. U.S. Pat. Nos. 5,008,228, 5,086,025 and 5,147,949 generally describe forming a dry supported catalyst system by the addition of trimethylaluminum to a water impregnated silica to form alumoxane in situ and then adding the metallocene. U.S. Pat. Nos. 4,808,561, 4,897,455 and 4,701,432 describe techniques to form a supported catalyst system where the inert carrier, typically silica, is calcined and contacted with a metallocene(s) and an activator/cocatalyst component. U.S. Pat. No. 5,238,892 describes forming a dry supported catalyst system by mnbdng a metallocene with an alkyl aluminum and then adding undehydrated silica. U.S. Pat. No. 5,240,894 generally pertains to forming a supported metallocene/alumoxane catalyst system by forming a metallocene/alumoxane reaction solution, adding a porous carrier, and evaporating the resulting slurry to remove residual solvent from the carrier.
While all these supported catalysts are useful, it would be desirable to have an improved metallocene catalyst system which in producing polymers does not foul the reactor. As used herein, the term "catalyst system" is equivalent to the term "catalyst composition". Particularly in a slurry or gas phase polymerization process, using these catalysts systems, there is a tendency for reactor operation problems during polymerization. During a typical gas phase polymerization process fines within the reactor often accumulate and cling or stick to the walls of a reactor. This phenomenon is often referred to as "sheeting". The accumulation of polymer particles on the reactor walls, the recycling lines and cooling system results in many problems including poor heat transfer during the polymerization process. Polymer particles that adhere to the walls of the reactor and can continue to polymerize and often fuse together and form chunks, which can be detrimental to a continuous polymerization process. Similarly, in a bulk liquid propylene polymerization process, using these catalyst systems there is a tendency for polymer particles to agglomerate and cling or stick to the walls of the reactor. This too can be detrimental to such a continuous process.
It would be highly desirable to have an improved polymerization catalyst system that in a polymerization process would significantly enhance reactor operability and provide an improved polymer product.